1. Field of the Invention
The present invention relates to a method and apparatus for sequencing an integrated circuit, and can be applied to integrated circuits designed for chip cards or circuit boards.
2. Discussion of the Related Art
Integrated circuits designed for chip card or circuit board type applications usually have an input designed to receive an external clock signal. This clock signal is used to generate an internal clock signal capable of ensuring the operation of the integrated circuit. Therefore, the internal operation of the internal circuit is sequenced with respect to the external clock signal.
For chip card applications, circuit board applications, or for any other application requiring high operating security, it is imperative to have the ability to prevent a situation where the presence of parasites on the external clock signal could disturb the internal clock signal and modify the operation of the integrated circuit. A disturbing action by parasites would be one way of carrying out fraudulent activity.
It has become common practice to use a filtering circuit in the internal clock signal generator. This filtering circuit is capable of overcoming disturbances of this kind in the external clock signal. An example of such a generator is described in the French patent No. 2 708 809. Structures also exist that can be used to obtain an internal clock signal with characteristics that depend little on variations in temperature or manufacturing method. A structure of this type is described for example in the patent No. FR 2 707 058.
However, it has been discovered that such structures, while they make it possible to prevent the disturbing of the internal operation of an integrated circuit, do not however make it possible to prevent another type of fraud, namely the observation of the behavior of the integrated circuit. Since the internal clock signal is based on the external clock signal, this external clock signal may be used as a synchronization signal. This would, for instance, enable an ill-intentioned individual in particular to obtain confidential data or even information by which the individual could reconstitute the program performed by the integrated circuit.